The present invention relates to noise reduction in a system comprising a digitizer and, more particularly, but not exclusively to noise reduction in a system comprising a digitizer associated with a display screen.
U.S. Pat. No. 6,690,156 “Physical Object Location Apparatus and Method and a Platform using the same” assigned to N-trig Ltd, and U.S. patent application Ser. No. 10/649,708 “Transparent Digitizer” also assigned to N-trig Ltd, describe a positioning device capable of detecting multiple physical objects, preferably styluses, located on a flat screen display. One of the preferred embodiments in both patents describes a system built of transparent foils containing a matrix of vertical and horizontal conductors. In one embodiment the stylus includes a passive resonance circuit, which is triggered by an excitation coil that surrounds the foils. The stylus is excited at a predetermined range of frequencies depending on the capacitance and inductance of the resonant circuit. Other embodiments may include a different kind of EM stylus. The exact position of the stylus is determined by processing the signals that are sensed by the matrix of horizontal and vertical conductors.
Existing digitizer systems use several noise removal methods to improve the detection precision. For example the received signal is processed through a band pass filter leaving a window of frequencies including the stylus frequency. The filtered signal may then be passed through a Fourier transform selecting the single frequency of the stylus.
Elements that induce an equal amount of noise on each conductive line regardless of the line location may then be eliminated through the use of differential amplifiers. For example, objects that are far enough from the sensor will have the same effect on all the sensor lines.
There are other examples of noise reduction methods that do not eliminate noise at the stylus frequency.
Using the various prior art systems, much of the noise is removed, but one element of noise necessarily remains because it cannot be identified and filtered out and that is noise that is at the same frequency as the stylus.
Preferred Application
The preferred application to which the embodiments to be described hereinbelow are applicable is a transparent digitizer for a mobile computing device that uses a flat panel display (FPD) screen. The digitizer detects the position of one stylus at a very high resolution and update rate. The stylus is used for pointing, painting, writing (hand write recognition) and any other activity that is typical for a stylus. The digitizer supports full mouse emulation. As long as the stylus hovers above the FPD, a mouse cursor follows the stylus position. Touching the screen stands for left click and a special switch located on the stylus emulates right click operation.
The application may utilize a passive EM stylus. External excitation coils that surround the sensor are utilized to energize the stylus. However, other versions may include an active stylus, battery operated or wire connected, which does not require external excitation circuitry.
In one application the electromagnetic object responding to the excitation is a stylus. However, other embodiments may include other physical objects comprising a resonant circuit or active oscillators, such as gaming pieces. Applications describing gaming tokens comprising resonant circuits are described in U.S. Pat. No. 6,690,156 (“physical object location apparatus and method and a platform using the same”).
In the preferred application, the digitizer can detect simultaneous and separate inputs from an electromagnetic stylus and a user finger. Hence, it is capable of functioning as a touch detector as well as detecting a stylus. However, other embodiments may include a digitizer capable of detecting only an electromagnetic stylus.
In a preferred application, the stylus supports full mouse emulation. However, in different applications the stylus could support additional functionality such as an Eraser, change of color, etc. In other embodiments the stylus could be pressure sensitive and changes its frequency or changes other signal characteristics in response to user pressure.
In a preferred application, the mobile device is an independent computer system having its own CPU. In different embodiments the mobile device might only be a part of system such as a wireless mobile screen for a Personal Computer.
In a preferred application, the digitizer is integrated into the host device on top of the FPD screen. In additional application the transparent digitizer can be provided as an accessory that could be placed on top of a screen. Such a configuration can be very useful for laptop computers, which are already in the market in very large numbers. Such systems can turn a laptop into a powerful device that supports hand writing, painting or any other operation enabled by the transparent digitizer.
In a preferred application, the digitizer supports one stylus. However, in different applications more than one stylus may operate simultaneously on the same screen. Such a configuration is very useful for entertainment application where multiple users can paint or write to the same paper-like screen.
In one application, the digitizer is implemented on a set of transparent foils. Alternatively such a digitizer may be implemented using either a transparent or a non-transparent sensor. One example is a Write Pad device, which is a thin digitizer that is placed below normal paper. In this example, the stylus combines real ink with electro magnetic functionality. The user writes on the normal paper and the input is simultaneously transferred to a host computer to store or analyze the data.
An additional example of a non-transparent sensor is an electronic entertainment board. The digitizer, in this example, is mounted below the graphic image of the board, and detects the position and identity of gaming figures that are placed on top the board. The graphic image in this case is static, but it could be manually replaced from time to time (such as when switching to a different game).
In some applications a non-transparent sensor could be integrated in the back of a FPD. One example for such an embodiment is an electronic entertainment device with a FPD display. The device could be used for gaming, in which the digitizer detects the position and identity of gaming figures. It could also be used for painting and/or writing in which the digitizer detects one or more styluses. In most cases, a configuration of non-transparent sensor with a FPD will be used when high performance is not critical for the application.
Technical Description
Transparent Digitizer
A preferred digitizer allows for the location and identification of physical objects, such as styluses and user's fingers. Identifying the location of the physical objects is sensed by an electro magnetic transparent digitizer that is mounted on top of a display. The transparent digitizer is described in U.S. Pat. No. 6,690,156 and detailed in U.S. patent application Ser. No. 10/649,708.
The various components and functionality manner of the transparent digitizer are as follows.
Sensor
In the preferred digitizer, the sensor is a grid of conductive lines made of conductive materials, such as ITO or conductive polymers, patterned on a transparent foil or substrate. For further information please refer to U.S. patent application Ser. No. 10/649,708, sub-heading: “Sensor”, the contents of which are hereby incorporated herein by reference.
Front End
In the preferred digitizer the Front end is the first stage where sensor signals are processed. Differential amplifiers amplify the signals and forward them to a switch, which selects the inputs to be further processed. The selected signal is amplified and filtered by a filter & amplifier prior to sampling. The signal is then sampled by an A2D and sent to a digital unit via a serial buffer. For further information please refer to U.S. patent application Ser. No. 10/649,708, under the heading “Front end”, the contents of which are hereby incorporated by reference herein.
Digital Unit
In the preferred digitizer the digital unit functions as follows: The front-end interface receives serial inputs of sampled signals from the various front-ends and packs them into parallel representation. A processing unit, such as a DSP core or a processor, which performs the digital unit processing, reads the sampled data, processes it and determines the position of the physical objects, such as stylus or finger. The Digital unit can be embedded in an ASIC component. The calculated position coordinates are sent to the host computer via link. For further information please refer to subheading: “Digital unit” in U.S. patent application Ser. No. 10/649,708, the contents of which are hereby incorporated by reference.
Detector
The detector consists of the digital unit and the Front end.
Detection of Stylus
The preferred digitizer utilizes a passive electromagnetic (EM) stylus. The stylus comprises two main sections; the first section is an energy pick-up circuit and the second section is an active oscillator which is coupled to the stylus tip. An external excitation coil that surrounds the sensor supply energy to the energy pick up circuit. The received energy is transferred to the active oscillator through a rectifying component such as a diode bridge. The exact position of the stylus is determined by the detector, which processes the signals sensed by the sensor. In the preferred embodiment only the electric wave of the electromagnetic signal generated by the stylus is utilized. However, other embodiments may utilize the magnetic portion in addition or instead of the electric wave. For further information please refer to U.S. patent application Ser. No. 10/649,708 assigned to N-trig, and US provisional patent application “Electromagnetic Stylus for a Digitizer System” filed December 2004, also assigned to N-trig, the contents of both applications are hereby incorporated by reference.
In the preferred digitizer, the basic operation cycle consists of windowing, FFT/DFT, peak detection, interpolation, filtering and smoothing. For further information please refer to U.S. patent application Ser. No. 10/649,708, sub-title: “Algorithms”.
Noise Sources
There may be a variety of noise sources in the stylus frequency range. The most common signals interfering with the stylus signals are signals that originate from conductive objects, such as a user finger, touching the screen. FIG. 1 is an electrical equivalent of a user finger touching one of the digitizer's antennas. When the user touches an antenna 11 a capacitance 12 is formed between the finger and the sensor conductors.
The noise situation is best explained with respect to finger induced noise signals.
There are two main scenarios that cause finger induced signals—
1. When the system is not connected to the common ground, electrical network vibrations lead to system oscillations 10 in reference to the ground. Since the user's body is not oscillating, the capacitance 12 between the user's finger and the system induces leakage current 13 through the user's finger to the ground.
2. When the user's body is subjected to electromagnetic interferences from the environment, it, and any associated finger, oscillates in reference to the system; as a result a leakage current is induced from the user's finger to the conductive antennas.
In both cases, the digitizer senses a leakage current originating from the user touching the sensor. When the leakage current induces a signal that is at the same frequency of the stylus, the leakage current can be mistaken for a stylus signal.
A second possible source of noise is the electronic components within the system, which radiate at many frequencies. These components may induce noise signals at the stylus frequency; thus interfering with stylus detection. Electronic devices placed in proximity to the system, such as cellular phones, may also radiate in frequencies that interfere with the stylus detection.
FIG. 2 is an example of a noise and stylus affecting the sensor at the same time. In this case the noise source is the user's finger touching the screen. The sensor 20 comprises a matrix of conductive lines. When stylus tip 21 is present at the surface of the sensor it affects the antennas in its proximity. One or more antennas in proximity to the stylus may suffer noise signals induced by a finger 22 touching the sensor. For example, antenna 23 exhibits signals induced by both stylus 21 and finger 22.
Erroneous Stylus Detection
In a digitizer of this kind, the stylus detection comprises two detection steps. The first step is to find the antenna exhibiting the maximum stylus signal. The second step is calculating the stylus position by interpolating the signals on the maximum signal antenna and its surrounding antennas.
A digitizer system designed to detect an electromagnetic stylus may suffer from two kinds of problems. The first kind is when the unwanted signals are stronger then the stylus signals, thus interfering with the first detection step. In this case the digitizer system should sample and employ the noise removal algorithm on all the antennas in order to reveal the antenna exhibiting the maximum stylus signal.
Reference is now made to FIG. 3, which describes a case when a user's finger 22 touching the screen induces a stronger signal 33 than the stylus 21, causing the digitizer to mistake the finger for the stylus. As a result the digitizer chooses the wrong antennas for interpolation.
The second kind of problem is when the stylus signal is stronger than the finger-induced signal. However, an error in the stylus detection may still occur during the interpolation step of the detection. FIG. 4, to which reference is now made, describes a case when a user's finger 22 touching the screen induces a signal that causes the digitizer to miscalculate the stylus position 34. The user finger 22 induces a signal on one of the X axis antennas 31 while the stylus 21 is located closer to a different X axis antenna 32. The signal received on the stylus antenna 32 is weaker than the signal 33 received on the finger antenna 31. Hence, the digitizer will miscalculate the stylus position 34.
The object of the present invention is to solve both cases and eliminate noise above and below the level of the stylus signal.
There is thus a widely recognized need for, and it would be highly advantageous to have, a noise reduction system devoid of the above limitations.